Combustion system for soaking pits



Oct. 22, 1963 w. H. DAILEY, JR

COMBUSTION- SYSTEM FOR SOAKING PITS Filed April 2:5. 1962 TEMPERATURTRANSMITTE CON 19(FUEL MAIN) zzz r l/IF m w m m. m m m 4 9 S 3 U B M O Cm INVENTOR. WILLIAM H. DAILEY JR. BY

ATTORNEY United States Patent 3,107,904 CQMBUSTION SYSTEM FOR SOAKINGPITS William H. Bailey, in, Solon, Ohio, assignor to Midland- RossCorporation, Toledo, Ohio, a corporation of Ohio Filed Apr. 23, 1962,Ser. No. 189,487 1 Claim. (Cl. 26315) This invention relates to acombustion system for high temperature furnaces. The invention isparticularly well suited for soaking pits which are used in steel millsfor heating partially chilled or cold ingots to rolling temperature.

In the operation of such furnaces it is highly desirable to utilize theheat in the flue gases to preheat the combustion air supply for thesoaking pit combustion syscom. This is accomplished by means of asuitable recuperator having a flue product passage in heat transferrelationship with 'an air passage.

For a recuperator to be satisfactory for such an application it must becapable of withstanding the deleterious effects of the high temperaturecombustion products (initially of the order of magnitude of 2400 F.)which flow therethrough. Recuperators constructed of suitable ceramicmaterials such as those of the type disclosed in United States Patent1,404,721 to Stein, have been proven extremely well suited for suchsoaking pit applications. In the operation of such a recuperator theflue products from the soaking pit generally pass through the flueproduct passage thereof at some sub-atmospheric pressure due to theinfluence of a draft producing means (e.g., a stack). To minimizeleakage or short-circuiting of combustion air flowing through the airpassage into the flue gas passage, it is also desirable to maintain thepressure of the air in the air passage at a subatmospheric value.Conventionally, this is accomplished by locating the air pumping meansdownstream rather than upstream of the recuperator, i.e. with the airpassage of the recuperator connected to the suction side rather than thepressure side of the air pumping means.

In such an arrangement of air pumping means and recuperator, however,the air pumping means is exposed to the maximum temperature of thepreheated air which, desirably, ought to be as high as possible.Accordingly, prior artisans have exhibited a preference forinspiratingtype pumping devices over mechanical devices such as fans,which are inherently more sensitive to high temperatures.

In an inspirating type pumping device the outlet of the air passage ofthe recuperator is disposed in communication with the inlet of a venturiwhose outlet or diffuser is in communication with the soaking pitcombustion system. A stream of high pressure entraining fluid is ejectedinto the venturi in such a manner as to entrain a stream of preheatedair from the recuperator. A drawback to prior art systems of this typeis that the entraining fluid is generally at some significantlylowertemperature than the temperature of the preheated air from therecuperator. Hence, the temperature of the mixture which is delivered tothe soaking pit combustion system is, undesirably, less than thetemperature of the air from the recuperator. This drawback was largelyovercome, however, in the system described in United States Patent2,627,398 to Hepburn, wherein a second recuperator (metallic) wasprovided in the flue product exhaust at a point downstream of theceramic recuperator for the purpose of preheating the stream ofhighpressure entraining fluid (also air). The Hepburn system served toincrease the temperature of the ultimate combustion air mixturedelivered to the combustion system in two ways; first, by virtue of thesensible heat added to the entraining fluid and second, by increasingthe entraining power of the entraining fluid, i.e., by reduc- 3,167,994.Patented Oct. 22, 1963 ing the quantity of entraining fluid required toentrain a given quantity of air from the ceramic recuperator. Inpractice, the system described in the aforesaid patent to Hepburn provedcapable of delivering preheated combustion air to a soaking pitcombustion system at temperatures from l 000 F. to 1500" F. In a typicalinstallation, 30%-35% of this air would consist of entraining airpreheated to 700 F. to 900 F. in a metallic recuperator while thebalance would consist of entrained air preheated to a temperature from1000 F. to 1700 F. in the ceramic recuperator.

The present invention is concerned with a novel inspirating-type pumpingsystem for delivering preheated combustion air from a ceramicrecuperator to a soaking pit combustion system which eliminates the needfor the metallic recuperator of the system described in the aforesaidpatent to Hepburn without unduly penalizing the efliciency of therecup-erative system. In the present invention it is proposed to utilizea high temperature high pressure stream of entraining fluid comprising amixture of products of combustion and air. This mixture is obtained froma combustor having an outlet disposed in fluid communication with theinlet of a venturi in inspirating relationship with preheated air fromthe out-let of the air passage of a ceramic recuperator.

The compressed air which is delivered to the combustor may be coldthereby obviating the need for the metallic recuperator of the jet pumpsystem of the aforesaid patent to Hepburn. The chilling effect andreduced entraining power of cold air jets of prior art jet pump systemsis obviated by the high temperature attained by reacting the compressedair with fuel.

In the preferred embodiment of the invention a single combustion airsupply system is provided for supplying preheated combustion air to aplurality or a battery of soaking pits. In such a situation a singlecombustion jet is utilized to draw air through a single ceramicrecuperator provided ina fiue common to all the pits and to deliver theair to a single manifold from whence it is delivered to each pit incustomary fashion. To insure proper operation of the burner meansprovided for each pit it is important in such an arrangement to maintainthe pressure of the preheated air in the combustion air manifold at arelatively constant value despite the varying air demands of theindividual pits. To accomplish this, it is necessary to provide: meansto modulate the quantity of preheated air drawn through the ceramicrecuperator in response to a signal indicative of the pressure of thecombustion air in the manifold. This, in turn, may be accomplished bycontrolling the operation of the jet pump combustor since the quantityof preheated air from the ceramic recuperator which will be entrained bythe jet of entraining fluid is a function both of the mass and thetemperature of the entraining jet.

It has been found that the most simple way in which the .combustor maybe feasibly operated to provide a jet of variable entraining effect isto modulate only the flow of fuel to the combustor While allowing theflow of compressed air to remain at a constant value thereby providing ajet of entraining fluid of relatively constant mass and variabletemperature. Without elaborate compensa'ting equipment, it is notfeasible to modulate both the flow of fuel and air to the combustorsince the ratio of entrained fluid to entraining fluid decreases as theflow of entraining fluid decreases in a system Where the jet pump mustoperate at constant pressure rise. The consequence of thischaracteristic is that the oxygen content of the mixture of entrainingfluid and entrained fluid delivered to the manifold would, undesirably,decrease significantly at lower flow values due to the increasedimportance of the effect of oxygen consumption by-the fuel streamdelivered to the combustor. shown that the oxygen content of the airdelivered to It has been.

the preheated air manifold in a system where only combustor fuel flow ismodulated will remain within the range of 19% to for the entire range offlow rates, whereas in a system wherein both the flow of fuel and air tothe combustor are modulated the oxygen content of the preheated air canvary over a range extending from 13% to 20% without the necessarycompensating equipment.

For a further considenation of what is considered to be novel andinventive attention is directed to the following portion of thespecification, the drawing, and the appended claim.

In the drawing:

I FIG. 1 is a plan view of a plurality of soaking pits, shown as three(3) in number for purposes of illustration, having a common system forsupplying preheated air cogstructed in accordance with the presentinvention; an

FIG. 2 is a sectional view taken on line 2-2 of FIG. 11 with appropriatecontrol instrumentation added.

Referring more particularly to FIG. 1, numerals 11, 12, and 13 representindividual soaking pits for reheating for rolling a plurality ofpartially chilled or cold ingots, indicated in outline by discontinuouslines in FIG. 2. Each pit comprises wall means forming a combustion andheating chamber 15 which is normally closed by a removable cover 16.Heating flame enters chamber 15 through a firing port 17 whose inlet iscoincident with the outlet of a burner 18. Fuel is delivered to burner18 from a suitable source, such as main 19, by means of conduit ll whichhas a flow control valve 22. Preheated combustion air is delivered toeach burner by a conduit 24 having a flow control valve 25.

.Fl'ue gas (products of combustion) is vented from each chamber 15 bymeans of an outlet port 26 to a flue gas exhaust manifold 27 which iscommon to all the soaking pits. Flue gases from exhaust manifold 27 aredrawn through exhaust duct 28 toward stack 29 by the draft createdthereby and are thence exhausted into the atmosphere.

Disposed within exhaust duct 28 is an indirect heat exchanger orrecuperator 31 having an air passage in indirect heat exchangerelationship with flue gases flowing therethrough. Recup-erator 31 mustbe constructed of suitable refractory materials, as in the mannerdescribed in United States Patent 1,404,721 to Stein, because availablemetallic materials are not as well suited to withstand the deleteriouseffects of the high temperature of the flue gases flowing therethrough.Air is drawn from the atmosphere into inlet housing 32 of recuperator 31through inlet port 33 by virtue of the draft produced by a jet pumpcomprising a nozzle 34 adapted to inject a stream of motive fluid intoventuri tube 35, which has an inlet portion 36 disposed in air outlethousing 37 of recuperator 3-1 in fluid communication with preheated air.The mixture of motive fluid from nozzle 34 and preheated air fromhousing 37 is delivered by venturi tube to air manifold 23 fordistribution to the individual pits.

The motive fluid which is delivered to venturi tube 35 by nozzle 34comprises products of combustion from a combustor 38 which is disposedin fluid communication with nozzle 34. It is to be expressly noted thatthe term products of combustion, as used in this regard, is intended toinclude a mixture of combustion products and air in excess of thatrequired for combustion. Fuel for combustion is delivered to combustor38 from fuel main 19 by means of cornbustor fuel conduit 39. Air isdelivered to com'bustor 38 at a. moderate pressure, preferably of theorder of magnitude of 3 p.s.i.g., fro-m a blower 41 by means of acombustor air conduit 42.

In the operation of the system, it is desirable that the combustionsystem for each pit be capable of operation independent of influence ofthe operation of other pits.

To achieve this, it is necessary that the pressure eithe mixture inmanifold 23 be maintained at a constant value. Accordingly, controlmeans are provided to control the operation of the jet pump in responseto a signal from pressure transmitter 43 indicative of manifold pressureas sensed by element 44. Accordingly, a flow control valve 45 isprovided in combustor fuel conduit 39 to control the flow of fuel tocombustor 38 in a manner that is an inverse function to the signal frompressure transmitter 43. It is noted that the flow of air from blower 41to combustor 38 is independent of the rate of fuel. It is desirable thatthe air and fuel flow to combustor 38 be controlled in this mannerbecause of the inherent characteristic of jet pump devices. to requirean increased ratio of motive fluid to entrained fluid when operated atreduced flow against a constant pressure. An increase in the ratio ofmotive fluid to entrained fluid would cause an undesirable decrease inthe oxygen content of the resultant mixture delivered to manifold 23,were it not for the increase of the percentage of oxygen in the motivefluid which results from the increased ratio of air to fuel consumed bycombustor 38 when only fuel flow is throttled.

The flow of fuel and preheated air to burner 13, nolike the flow of fueland air to combustor 35, are controlled in ratio to each other by meansof flow control valves 22 and 25. One of valves 22 and 25 is operated bya signal from temperature controller 46 so as to control flow throughthe respective conduit in a manner that is an inverse function of thesignal transmitted from temperature transmitter 47 to controller 46,which, in turn, is proportional to temperature within chamber 15, assensed by thermal element 48. The other of valves 22 and 25 iscontrolled in response to a signal from ratio control device 49- which,in turn, is responsive to a signal from air flow transmitter 51,indicative of air flow through conduit 24, and a signal from fuel flowtransmitter 52, indicative of fuel flow through conduit 21.

As will be evident to those skilled in the art, various modificationsand alternatives can be made to the embodiments of this inventionwithout departing from the spirit or scope of the disclosure or theclaim.

I claim:

'In soaking pit apparatus having wall means defining a plurality ofcombustion and heating chambers, a burner associated with each chamberfor delivering heating flame thereto, a single exhaust passage in fluidcommunication with each chamber for exhausting combustion productstherefrom, recuperator means disposed in said exhaust passage and havingan air passage in indirect heat transfer relationship with the flow ofcombustion products through the recuperator, improved jet pump means fordrawing heated air through said recuperator and delivering heated air tothe burner associated with each of said chambers comprising, incombination: a manifold for receiving combustion air; a venturi tubehaving a diffuser portion in fluid communication with said manifold andan inlet portion communicating with the outlet of the air passage of therecuperator; a combustor adapted to promote the combustible reaction ofa stream of fuel and a stream of air to produce a stream of products ofcombustion, said combustor being disposed in fluid communication withthe inlet portion of the venturi tube whereby a stream of products ofcombustion will flow from the combus-tor into the venturi tube ininspirating relationship with preheated air from the recuperator;com-buster fuel and air conduit means for delivering streams of fuel andair to said combos-tor for reaction; means sensitive to the pressurewithin the manifold for controlling the rate of flow of fuel to saidcombustor independently of the rate of flow of air in a manner adaptedto maintain manifold pressure relatively constant; a burner fuel conduitassociated with each of said chambers for deliveringfuel to the burnerassociated therewith; a burner air conduit associated with each of saidchambers for delivering air from the manifold to the burner 'associatedwith the respective chamber; and means responsive to temperature withineach chamber for controlling the flow of fuel and air :to the respectiveburner through the respective burner fuel conduit and burner -airconduit.

6 References Cited in the file of this patent UNITED STATES PATENTS1,717,115 McCann June 11, 1929 5 2,504,320 Gamble Apr. 18, 1950 FOREIGNPATENTS 766,603 Great Britain Jan. 23, 1957

